The reliability of a rolling mill is primarily dependent upon the maintenance as well as the proper alignment of the components included therein. Improper roll and roll chock conditions can so adversely affect bearing performance and rolling mill practices that both down time as well as product quality may be adversely affected. At the present time the proper alignment of equipment components is becoming more of a factor as competitive pressure force rolling mills to increase their productivity of both hot and cold rolled product. Specifically, this means increases the mill loads, running at higher speeds, and using work roll or backup roll bending for accurate shape control. It is obvious that these operating conditions subject the backup rolls, work rolls and the bearing chocks to increasing demand. Maintenance of all of these items in a proper and effective manner is an absolute necessity to achieve increased mill productivity.
In a typical rolling mill both the backup rolls and the work rolls have center lines which are substantially parallel to each other. Ideally, the center lines of the top and bottom work rolls are on a common vertical line as is the center line of the backup rolls. In some cases a slight horizontal offset is incorporated between the center lines of the work rolls and the center lines of the backup rolls. Typically, this offset is in a direction toward the exit side of the mill. However, even under such situations all of the center lines of the work rolls and the backup rolls should be substantially parallel to one another.
When a strip rolling mill operates with the roll center lines in this fashion and equal or even strip rolling pressure exists across the width of the mill, the operation of the work roll bearings contained within the chock is nearly ideal. A substantial radial load on the bearings is expected due to the roll balance load and/or work roll bending loads. Other bearing loads which might occur are due to differences in strip tension between the entry side of the mill and the exit side of the mill. These forces, however, can be predicted and accommodated for in the original design of the mill. Unexpected loads on the bearings, chocks and the rolls themselves can be anticipated but not normally accommodated for in a crossed-roll condition.
A crossed-roll condition which causes serious work roll problems and backup roll problems to the bearings and product quality exists when the vertical center line of the top and bottom work roll necks are not in exact vertical relationship to each other and are not in a parallel relationship across the width of the mill. The positioning error on one side of the mill may not occur on the opposite side of the mill. If this positioning error becomes significant, the mill operator may detect a widening of the roll gap on one side of the mill and may correct for the situation by adjusting only one mill screw to achieve a closer gap. In such a case the quality of the product may not be adversely affected by a crossed-roll condition but undesirable wear to the rolls, bearings and chocks can be expected.
When the crossed-roll condition becomes exaggerated, there may in fact be an adverse affect to the quality product as well as to the life of the rolls, bearings and chocks. When the strip product enters the mill, it forms an intermediate direct contact with the two work rolls and, due to the slightly divergent surface direction of the mated rolls, tend to push each of the rolls away from the other. This interaction between the rolls generates axial forces which tend to push them out of the mill and may result in substantial wear, fracture and maintenance problems to the mill in general and to each of the above noted elements specifically. In addition, such a condition would result in a product quality which is less than desirable.
The massive forces needed to counteract the generated thrust load are applied to the work roll bearings and chocks on the operator side of the mill. The magnitude of the generated thrust force is primarily influenced by the magnitude of the mill separating force and the degree of the positioning error that results in crossed rolls. If a mill is operated with a severe crossed-roll condition, the work roll thrust load may be ten times the work roll thrust loads normally expected when such rolls are operated in a substantially parallel direction of rotation. These increased thrust loads would result in a reduction in the life of the bearings and chocks in some cases of several hundredths of the normal life expectancy of these elements.
Normal maintenance and setup procedures used in the past have relied upon the alignment of the center lines of each of the rolls through a procedure which involves the completed breakdown of the chock components, including the bearings, and the measurement of distances and tolerances from the inside of the chock toward the outside lateral support faces. While not only being time consuming, these practices become expensive and are not indicative of the spacial orientation of the lateral support faces of the chocks relative to the axis of rotation. It therefore becomes apparent that a substitute method for the calculation of the positioning of the lateral supportive chock faces with regard to the center line of the associated roll would be advantageous in the proper maintenance and alignment of the rolls within the mill. Such a method would not only obviate the need for the disassembly of the roll and chock, but would provide a more reliable and efficient method for determining such spacial orientation for the proper alignment of the roll center lines.